1. Field of the Invention
The present invention relates to planar sonar arrays. More particularly, the invention relates to the arrangement of transducer elements in planar sonar arrays.
2. Description of the Prior Art
Conventional planar sonar array designs typically comprise ceramic or polyvinylidene fluoride (PVDF) transducer elements arranged in a single plane according to a required inter-element spacing. The inter-element spacing is based on the half wavelength dimension of acoustic signals at the desired operating frequency. There are several disadvantages associated with this conventional array design. First, the inter-element spacing places an upper bound on the center to center distance between adjacent elements. If this upper bound is violated, beamformed acoustic energy will be allowed to fold back spatially, resulting in false indications of received energy. This is analogous to frequency domain aliasing where the Nyquist criteria has been violated, but in the case of a beamformer or spatial filter, the independent variable is bearing instead of frequency.
Furthermore, because the inter-element spacing constrains the maximum separation distance between elements, there is a corresponding upper bound on element size. That is, as the inter-element spacing is decreased, the area available for the element face decreases as a function of the square of the linear dimension of the element's side. Element signal-to-noise ratio is proportional to the volume of an element and decreases as the elements get smaller. Furthermore, since the PVDF transducer elements used in conventional array construction are essentially flat plate capacitors, the capacitance of any element is directly proportional to the electrode area of the element. As the area available for the element electrode decreases, the capacitance is lowered, thereby increasing electronic noise floor levels. Elevated noise floor levels can mask acoustic signals of interest. Furthermore, the effects of the increased noise floor levels can be exacerbated by the fact that at certain operating frequencies, the deep ocean ambient sound pressure level has a notch making it the quietest region in the usable spectrum. It is at these frequencies that problems resulting from the limited capacitance of the elements are most noticeable.
In addition to the sensitivity and detection problems, elevated noise floor levels complicate the fabrication of conventional arrays. To keep the electronic noise floor level as low as possible, the capacitive loading of the elements must be kept at a minimum. The capacitive loading at the elements is kept at a minimum by keeping the lead lengths of any signal conditioners (preamplifiers) as short as possible. However, this requirement complicates fabrication and provides little space to connect the preamplifiers.
Two alternatives have been considered to increase the capacitance in a flat plate capacitor without increasing the area of the electrodes. One approach is to decrease the spacing between the two electrodes of the capacitor. The second approach is to increase the dielectric constant of the material between the electrodes. However, neither of these approaches has proven feasible for use in elements within a sonar array. The first approach decreased the element sensitivity greatly and alternative dielectric materials considered in the second approach either decreased the element sensitivity or increased the thermal noise of the array.
Thus, what is needed is a planar array with an arrangement for the individual elements that increases the capacitance of the elements by increasing their surface area while maintaining the inter-element spacing required to avoid spatial aliasing in the received signals.